Architectural, air-drying coating compositions such as paints, lacquers and varnishes commonly comprise three main components: an autoxidizable binder polymer, which is the film-forming component, a solvent, which is the carrier for the non-volatile components, and driers or siccatives, which influence the drying speed of the composition. Autoxidizable binder polymers can be dissolved in an organic solvent such as white spirit or hydrogenated white spirit. Alternatively, autoxidizable binder polymers can be dispersed in water.
Autoxidizable alkyd resins are long-established binder polymers for film-forming coating compositions acknowledged for their esthetic properties, low surface tension (which enables the wetting of and adhesion on a wide variety of substrates and facilitates pigment wetting), applicability by various techniques, and cost-effectiveness. Because of these properties, alkyd resins are the most widely used air drying binders in coating compositions. Autoxidizable alkyd resins comprise drying or semi-drying unsaturated fatty acids or oils, which are generally attached to the polyester backbone of polyols and polycarboxylic acids. When the coating composition is applied to a substrate, the drying process starts by solvent evaporation and the binder polymers undergo autoxidation and subsequently form cross-links between the polymer chains resulting in a solid and coherently dried film. The drying process of autoxidizable architectural coating compositions takes place at ambient temperatures ranging from 0 to 40° C., whereby the presence of oxygen is essential. Since the drying process proceeds slowly, the chemical conversion of alkyd resins is habitually catalyzed by salts of metal ions as catalytic oil drying agents. These salts of metal ions act as driers or siccatives.
These metal salts, containing either alkaline-earth metals or heavy metals, affect the autoxidation of the binder with air and/or catalyze cross-linking of the unsaturated sites. The drying time can consequently be reduced from days to hours. The presence of efficient driers is therefore essential for the drying of air-drying coating compositions.
These catalytic oil drying agents are commonly divided into three main classes according to their catalytic activity: primary driers, which all possess significant catalytic activity, coordination driers, which promote the film-forming process by bridging two or multiple polymer chains, and secondary driers, which have little to no catalytic effect when used on their own but assist the drying process by activating the metal in the primary drier. A wide range of metals can form the basis of these driers, examples include but are not limited to cobalt (Co), manganese (Mn), iron (Fe), vanadium (V), cerium (Ce), zirconium (Zr), lanthanum (La), neodymium (Nd), aluminum (Al), bismuth (Bi), strontium (Sr), zinc (Zn), lithium (Li), calcium (Ca), potassium (K), barium (Ba) and lead (Pb). The metal salt ligands can also play an important role in drying speed: drying accelerators are organic ligands that are able to increase the activity of primary drier metals causing a more rapid drying of the coating film.
Primary driers (also called top driers, surface driers or oxidation driers), promote the top-down hardening of a liquid alkyd resin. The mode of action of the primary driers in the autoxidative curing process is deactivation of natural occurring anti-oxidants, oxygen uptake, peroxide formation and peroxide decomposition. Primary driers are characterized by having at least two accessible valence states which allows catalytic hydroperoxide decomposition and regeneration of the active species. Examples of primary driers are cobalt (Co), cerium (Ce), lead (Pb), iron (Fe), manganese (Mn) and vanadium (V) carboxylates. To enhance homogeneous through drying of a coating film, primary driers are frequently used in combination with secondary and coordination driers.
Coordination driers, also referred to synonymously as through driers, promote the film-forming process by interaction with the carboxyl and hydroxyl groups in the polymeric binder. This way, coordination driers can bridge two or more polymer chains. These carboxyl and hydroxyl groups may be initially present in the binder molecule or formed during the autoxidation process. This group comprises the metal driers based on zirconium (Zr), strontium (Sr), aluminum (Al), bismuth (Bi), lanthanum (La), and neodymium (Nd).
Secondary driers are also referred to synonymously as auxiliary driers. These metal driers exist in a single oxidation state and are not catalytically active by themselves. However, secondary driers do affect the rate-of-dry by interacting with the primary driers. Secondary driers include calcium (Ca), barium (Ba), potassium (K) and lithium (Li) metal soaps.
To improve the appearance and quality of the coating film and to accelerate the drying time, in a typical alkyd formulation the primary drier is combined with auxiliary driers such as zirconium (Zr), strontium (Sr), aluminum (Al) (as disclosed in EP0148636), neodymium (Nd) (as disclosed in U.S. Pat. No. 5,154,764) and bismuth (Bi) (as disclosed in U.S. Pat. No. 4,331,575) and secondary driers such as calcium (Ca), barium (Ba), lithium (Li) and potassium (K) (as disclosed in U.S. Pat. No. 4,311,625).
K-octoate is known to activate Co-based driers but because of its hydrophilic character this drier is mainly used in aqueous coating formulations.
The most widely used primary driers are cobalt carboxylates because of their good drying performance at ambient temperature and coloristic properties. However, they will most likely be restricted in the near future because of regulation issues.
Hence there is an increasing demand for alternative, non-cobalt based driers. Alternative driers are based on vanadium (V) (as disclosed in WO2009007288), manganese (Mn) (as disclosed in EP1382648), a combination of cerium (Ce) and manganese (Mn) (as disclosed in EP1394230) and iron (Fe) (as disclosed in WO03/093384). However these alternative driers are not as active as cobalt driers and are generally colored. Recently developed curing agents for air-drying alkyd resins based on iron or manganese complexes (as disclosed in WO2008003652 and US2009253833) show a highly efficient drying behavior under ambient conditions. However, the curing agents based on iron or manganese complexes do not show a comparable drying activity to cobalt salts under conditions of low temperatures or do not show a comparable activity with regards to through hardening.
Thus, there exists a need to improve the drying performance of non-cobalt primary drier compositions.